The present invention relates to a semiconductor device and a method for fabricating the same and, more particularly, to a semiconductor device in which a tungsten plug is provided on a semiconductor element or a wire and a method for fabricating the same.
Conventionally, a tungsten plug provided to extend through an insulating film has been used as an electrode for supplying a current and a signal to a semiconductor element or a lower layer wire covered with the insulating film (see, e.g., Japanese Laid-Open Patent Publication No. 2001-60564).
FIG. 9 shows a cross-sectional structure of a conventional semiconductor device having a tungsten plug. As shown in FIG. 9, an insulating film 102 made of a silicon oxide is formed on a semiconductor substrate 101 made of silicon and having an upper surface provided with a semiconductor integrated circuit (not shown) including a semiconductor element and the like. The insulating film 102 is formed with a contact hole 102a reaching the semiconductor element. A titanium film 103, a titanium nitride film 104, and a first tungsten film 105 are stacked successively in layers on a wall surface of the contact hole 102a. A second tungsten film 106 is provided on the first tungsten film 105 in such a manner as to fill up the inside of the contact hole 102a. 
In the conventional semiconductor device, the first tungsten film 105 has the function as a seed layer for the crystal growth of the second tungsten film 106 serving as the main body portion of the plug in the contact hole 102a, while each of the titanium film 103 and the titanium nitride film 104 has the function as an adhesion layer for enhancing the adhesion between the first tungsten film 105 and the insulating film 102.
In a method for fabricating the conventional semiconductor device, the insulating film 102 is formed first on the semiconductor substrate 101 and then the contact hole 102a is formed in the insulating film 102. Then, the titanium film 103 and the titanium nitride film 104 are deposited successively over the entire surface of the insulating film 102 so that the wall surface and the bottom surface (i.e., the upper surface of the semiconductor substrate 101 exposed at the bottom of the contact hole 102a) of the contact hole 102a is covered with the titanium film 103 and the titanium nitride film 104.
Next, the first tungsten film 105 is formed on the titanium nitride film 104 by CVD (chemical vapor deposition) using the reduction of a tungsten fluoride by a silicon hydride such that the contact hole 102a is halfway filled therewith. Subsequently, the second tungsten film 106 is formed on the first tungsten film 105 by CVD using the reduction of a tungsten fluoride by hydrogen such that the contact hole 102a is filled up completely therewith.
Thereafter, the respective portions of the second tungsten film 106, the first tungsten film 105, the titanium nitride film 104, and the titanium film 103 located outside the contact hole 102a, i.e., the respective portions of the second tungsten film 106, the first tungsten film 105, the titanium nitride film 104, and the titanium film 103 deposited over the insulating film 102 are removed successively by CMP (chemical mechanical polishing), whereby a plug composed of the first tungsten film 105 and the second tungsten film 106 is formed in the contact hole 102a. 
In accordance with the method for fabricating the conventional semiconductor device, however, a problem is encountered that a tungsten-film burying defect occurs to form a seam (void) 107 in the contact hole 102, as shown in FIG. 10, which leads to the degradation of the reliability of the semiconductor device resulting from the defect. The problem becomes more pronounced with the miniaturization of the contact hole or with the increase of the aspect ratio thereof.